1. Field of the Invention
This invention relates to copper particle clusters and powder containing the same suitable for use as the conductive filler of a conductive paste.
2. Background Art
In the fabrication of thick-film circuit boards and the like by screen-printing a conductive paste on an insulating board, the silver-system paste long used as the main conductive paste has recently been replaced to some extent by copper-system paste. Copper-system paste has certain advantages over silver-system paste, including higher resistance to migration, excellent soldering tolerance, and low cost. A copper-system conductive paste having these advantages is obtained by dispersing copper powder of a particle diameter of about 0.1-10 xcexcm in a vehicle (resin binder).
Copper paste using copper powder as metallic filler has also drawn particular attention in connection with the recent practice of fabricating multi-layer boards for high-density component mounting by forming stacked boards with through-holes or via holes in the shape of fine holes or slits, charging conductive paste into these openings, and solidifying the conductive paste by heating to form high-density conductive circuits in the boards. Copper paste can be used to advantage as the conductive paste charged into the small hole-like or slit-like openings.
Known processes of producing copper powder include the mechanical pulverization process, atomization process of spraying molten copper, electrolytic cathode deposition process, vapor deposition process and the wet reduction process. Although each method has its merits and demerits, the wet reduction process is the main one used to produce copper powder for conductive paste because it enables relatively ready production of fine powder of a particle diameter suitable for use in a conductive paste. Copper powder production processes using the wet reduction method are taught by, for instance, Japanese Patent Publication JPA No.Hei 4-116109 (1992), JPA No.Hei 2-197012 (1990) and JPA No.Shou 62-99406 (1987).
Although copper-system paste rates highly in various aspects of performance, its most basic requirement for use as a conductive paste is excellent conductivity. When a metallic filler of a given purity is dispersed in a resin at a given filling rate, the electrical resistance of the resulting paste will nevertheless exhibit different values depending the particle size distribution and particle shape. In order to obtain a paste with low electrical resistance, it is obviously important to ensure close contact among the particles, i.e., to ensure that the metallic particles are dispersed in the resin at a high filling rate so as to increase the contact interface among the particles. Although obvious, this is hard to achieve in practice while also maintaining the other qualities required by the conductive paste, such as good viscosity property.
Low electrical resistance can generally be obtained when the metallic filler in the resin includes many particles with irregular surfaces because the surface irregularities increase the contact area among the particles. However, a paste containing a large percentage of particles with pronounced surface irregularities is high in viscosity and therefore difficult to charge into the through-holes or via holes. Simultaneous reduction of electrical resistance and viscosity is therefore difficult because an attempt to lower electrical resistance by controlling particle shape has the adverse effect of increasing paste viscosity.
A first object of the present invention is to provide copper particles/powder that exhibit high conductivity (low electrical resistance) when dispersed in a resin.
A second object of the present invention is to provide copper particles/powder that exhibit high conductivity (low electrical resistance) and minimize viscosity increase when dispersed in a resin.
The present invention achieves the first object by providing copper particle clusters for conductive paste individually composed of two or more unit particles, preferably 2 to 20 unit particles, joined through neck portions. The individual copper particle clusters for conductive paste provided by the present invention are preferably composed 2 to 20 unit particles of about 0.5-10 xcexcm diameter joined through neck portions in arbitrary directions in three-dimensional space. A xe2x80x9cneck portionxe2x80x9d is defined as a portion through which two unit particles are joined which is of a diameter smaller than the diameter of at least one of the unit particles joined thereby, preferably smaller than the diameters of both unit particles joined thereby.
A metallic copper powder composed of such copper particle clusters can be advantageously produced by a process for producing copper powder comprising a step of precipitating copper hydroxide by reacting an aqueous solution of a copper salt and an alkali to obtain a suspension containing copper hydroxide, an intermediate reduction step effected by adding a reducing agent to the suspension to reduce the copper hydroxide to cuprous oxide, and a final reduction step of reducing the cuprous oxide in the suspension to metallic copper using a reducing agent, in which process the copper hydroxide precipitating step is conducted under an atmosphere of an oxygen-containing gas, the copper hydroxide precipitating step is conducted in an aqueous solution of an Fe concentration of not greater than 50 ppm, and an oxygen-containing gas is blown into the suspension containing cuprous oxide after the intermediate reduction step.
The present invention achieves the second object by providing a copper powder for conductive paste (conductive filler) composed of copper particle clusters with neck portions and metallic particles without neck portions. More specifically, the invention provides a filler for conductive paste capable of simultaneously lowering electrical resistance and viscosity consisting essentially of a mixture of copper particle clusters A individually composed of two or more unit particles joined through neck portions and spherical metallic particles B of smaller diameter than the particles A. The spherical metallic particles B are preferably mixed with a copper powder comprising the copper particle clusters A at a rate such that the weight ratio of B to A (B/A) is in the range of 1/19 to 3/5, and the ratio of the average particle diameter DA of the copper particle clusters A to the average particle diameter DB of the spherical metallic particles B (DA/DB) is in the range of 5/4 to 8/1. The spherical metallic particles B can be copper particles or copper particles coated with silver. The average particle diameter DA of the copper particle clusters A is preferably in the range of 4-8 xcexcm. The copper powder comprising the copper particle clusters A is still more preferably one that has been subjected to surface smoothing treatment. The surface smoothing can be achieved by causing mechanical contact among the copper particle clusters A. When the copper particle clusters A are ones that have been subjected to surface smoothing treatment, the copper particle clusters may be present whose neck diameter is greater than the diameter of at least one of the unit particles at opposite ends of the neck portion. The second object can also be achieved in this case because the improved fluidity of the filler produced by the surface smoothing reduces the paste viscosity.